Knowledge of current fuel tank venting systems is essential in order to ensure the determination of the mass flow via the tank venting valve in internal combustion engines having a variable valve train or in working points with a subcritical pressure condition involving significant exhaust gas recirculation. In a sophisticated engine controller, which aims at maintaining certain fuel/air ratios in the combustion chamber of the internal combustion engine with great accuracy, the fuel tank venting system may not be operated independently thereof, particularly since the fuel vapors are supplied to the internal combustion engine. The controller of the fuel tank venting system and the controller of the internal combustion engine are therefore closely connected. The controllable valves of the engine are controlled as a function of certain operating conditions of the internal combustion engine and different states of the fuel tank system or the fuel tank venting system.
The throttle valve position, the air flow, or the pressure may be measured in the intake system. In the engine itself, the engine speed may be the object of the detected signals. Its oxygen content is normally also detected in the exhaust gas of the internal combustion engine. The signals received by the controller unit are implemented in programs for controlling the system components, so that feeding of the fuel vapors into the intake system takes place without impairing the operation of the internal combustion engine. Moreover, a check of the system efficiency is possible in this way.
In the devices according to the related art the fact is utilized that the mass flow, which flows into the intake manifold via the tank venting valve, is calculated using the outflow characteristic curve as a function of the pressure differential at the tank venting valve. The outflow characteristic curve has a great ascent in the area of a subcritical pressure condition which results in inaccuracies and in the worst case in instability in the calculation of the mass flow. In previous engine controller designs, the mass flow via the tank venting valve is calculated as a function of the throttle valve angle and the engine speed in order to improve the accuracy. It is assumed here that the intake manifold pressure is proportional to the mass flow flowing in via the throttle valve. This assumption is not fulfilled in systems having a variable valve train or an exhaust gas recirculation.